> geroldf@sdd.hp.com (Gerold Firl) wrote:> > >Here is an interesting question: did the genes for lactose tolerance> >diffuse outward from a single source, or did evolution locally select> >for it in areas where cattle were kept for meat? Actually, both> >processes undoubtedly occured, but which was faster?> > To me the more interesting question is, why did the lactose> intolerance develop? And when? And why was it adaptive?> > Best wishes. rs >

I hope someone can help me out with references on this subject but I
recall that the evolutionary biological hypothesis for the prevalence of
lactose intolerance among African and Mediterranean peoples (among others)
suggests that melanin (skin color) is a crucial factor.

The hypothesis suggests that in areas where people were able to synthesize
Vitamin D from sunlight (absorption by melanin) the necessity of digesting
milk for Vitamin D was obviated. A selective advantage accrued in
populations in which adults became lactose intolerant because limited milk
supplies would then be consumed primarily by children (who need it most)
instead of by the adults. Thus, the gene for lactose intolerance would
become more prevalent. (This may suggests that the gene could be selected
for independently in several distinct populations rather than necessarily
by diffusing out from one source). Also, the inability of adults to
consume milk would tend to limit the size of cattle herds which would have
a beneficial effect on resource use in marginal areas. (There wouldn't be
so many cows to accelerate desertification).

In Europe, the populations' lack of melanin (and long winters with
relatively little sunshine) meant that digestion of milk was necessary to
obtain Vitamin D and so any lactose intolerance genes would prove to be
detrimental and the gene would not become prevalent.